Question

In a simplified situation like this one, with 3 classes:

class ClassONE{

   protected List<IntermediatedClass>;

   (... getters / setters)

}

class ClassTWO

 protected String someData;

 (... getters / setters / iniciate List etc...)

}

class IntermediatedClass{

 public ClassONE one; //Points to an object (row) of ClassONE
 public ClassTWO two; //Points to an object (row) of classTWO.
 public Double data; //Data that need being saved with this particular relation between obj1 and obj2.

}

I know that with Hibernate you can declare a ManyToMany monodirectional relationship between class1 and class2. Class1 can have some objects of class2, and class2 can be on some objects of class1. Thats just [CLASS1] N —- N [CLASS2].

I want declare a [CLASS1]1—N[INTERMEDIATECLASS]N—1[CLASS2] typical N-N database implementation. Intermediate class has an attribute very important: data, a Double type. I cannot find the way to declare IntermediateClass as my own way to implement this N–N with this data in Hibernate Annotations.

Any help, please? Just with this simple example i can solve my trouble. I didnt would make my tries on it in order to maintain clean the code for your understanding.

Answer 1

You can create a full blown bidirectional many-to-many mapping with an intermediate mapping class with additional properties the same way you would design your ER model:

class A {
  private List<AB> ab;

  @OneToMany(mappedBy = "a")
  public List<AB> getAb() {...}
}

class B {
  private List<AB> ab;

  @OneToMany(mappedBy = "b")
  public List<AB> getAb() {...}
}

And the mapping class:

class AB {
  private A a;
  private B b;
  private double data;

  @ManyToOne
  @JoinColumn
  public A getA() {...}

  @ManyToOne
  @JoinColumn      
  public B getB() {...}

  @Column
  public double getData() {...}
}

But since bidirectional associations like this can get rather messy and hard to maintain in your application code I would do without the bidirectional association and just define the two unidirectional many-to-one associations in class AB.